1. Field of the Invention
The present invention relates to a circuit for the arcless de-energization of a direct current load receiving direct current from a rectified alternating current source and, more particulary, to an electric circuit for the arcless de-energization of direct current trolley systems used in an underground environment in mining operations.
2. Description of the Prior Art
In electric power distribution systems, it is common to employ circuit breakers to interrupt the flow of current from a current source, such as a direct current source, in the event of an overload on the system. A common problem that often accompanies the de-energization of a direct current load protected by a circuit breaker against overloading is the presence of arcing at the breaker as the breaker attempts to open to clear an overload on the system. In certain operations utilizing electrical power distribution systems, such as certain underground mining operations, it is particularly critical that the circuit breaker or other switching apparatus used to disconnect the load from the system in an attempt to clear the overload do so with a minimum of arcing. In an underground mining operation, for example, it is essential that the flow of current be interrupted by the safe operation of a circuit breaker, or other protective current interrupting apparatus, without destructive arcing. Arcless interruption of the flow of current in response to an overload is highly desirable in a mining environment, since arcless circuit interruption substantially minimizes the risk of equipment damage and maximizes electrical component life.
Various circuits and apparatus are known in the art for effecting circuit interruption in response to an overload. For example, U.S. Pat. No. 3,725,742 to Pollard discloses a static circuit breaker. In Pollard, a three phase, static circuit breaker has three phase switches, each of which has a pair of power thyristors connected in inverse parallel relationship with one another and a common commutation circuit. A commutating circuit is provided in each phase switch which includes a commutating capacitor, a commutating thyristor, and a commutating inductor. A common surge suppressing circuit for all the switches is also provided.
U.S. Pat. No. 4,045,887 to Nowell discloses a switching regulator control circuit. The control circuit includes silicon controlled rectifiers and inductive reactors. The control circuit provides fault detection and voltage regulation.
U.S. Pat. No. 4,042,966 to Newell et al. discloses a motor control circuit for controlling the operation of a motor having main running-speed windings and start-up windings. A fixed time delay circuit is provided to energize the main windings after a time delay following energization of the start-up windings. A lock-out circuit is additionally provided to prevent re-start of the circuit following a fault condition.
U.S. Pat. No. 4,042,965 to Wisman discloses a power interruption safeguard apparatus used in the prevention of undesirable automatic application of power to a load in the event of a power outage. Wisman's apparatus effects circuit interruption in a direct current control circuit portion of a direct current motor speed controller by means of a silicon controlled rectifier.
U.S. Pat. No. 3,911,352 to Slack discloses a blocking oscillator type power supply with power foldback short circuit protection. The blocking oscillator circuit includes a capacitor and a means for repetitively charging the capacitor at a rate which causes the oscillator to operate at a substantially lower frequency when the secondary winding of the blocking oscillator transformer is short circuited to limit the power dissipated in the oscillator.
U.S. Pat. No. 4,203,040 to Abbondanti et al. discloses a force commutated static isolator circuit to provide a bi-lateral static isolator which is activated by "soft" forced commutation. Two thyristors are connected in series opposition between two points in a power line to be interrupted. This patent mentions that a resonant circuit is applied to the conducting thyristor in order to turn it off with a relatively small reverse voltage obtained by resonant discharge of a precharged capacitor through an inductor and a diode connected in reverse with the thyristor.
U.S. Pat. No. 4,227,145 to Bonikowski et al. discloses an apparatus for detecting faults in electric cables. The apparatus includes a solid state switch unti having two thyristors arranged in an inverse-parallel configuration and continuously conducting current up to a pre-set maximum and becoming non-conductive after conducting a current in excess of the pre-set maximum for a pre-set period.
What is needed, therefore, especially in underground mining operations, is a circuit for the arcless de-energization of a direct current load. What is further needed is a circuit for the arcless de-energization of a direct current load capable of minimizing the risk of equipment damage when de-energization of a direct current load is required by the presence of an overload condition.